Do you want them to have a different username for every post? Can’t you just get cogito to give them a username that you deem safe?
Is it unimportant to be able to connect a user to their previous posts? If it is important, can’t you let them choose a name that they deem safe?
If you have a budget, I’d be happy to write a plugin that fits your needs and budget. It could change the username of new users to a random one that fits your requirements, for example.